Nuclear fuel assemblies for power reactors are designed and constructed for achieving the best possible compromise between requirements which are to a large extent contradictory. Neutron absorption by the material forming the structure and guiding the flow must be as low as possible. That leads to forming the sheath of the elements from a material having a low neutron capture cross-section and reducing as much as possible the skeleton supporting the elements of the assembly. In particular, the use of a solid peripheral casing about the bundle has been successfully dispensed with and the skeleton reduced practically to two end nozzles connected together by elongated elements and to grids spaced apart along the bundle and defining individual cells or openings for the elements.
Another result sought during design of the assemblies is to make the temperature of the sheaths as homogeneous as possible, i.e., to reduce the hot spots. For that purpose, the coolant flow should be homogeneous throughout the whole of the assembly. Mixing fins are provided to homogenize the flow and to reduce the temperature differences by causing a transverse redistribution of the coolant streams. But the fins should not increase the head loss to the extent that there would be an inacceptable reduction of the flow rate. Numerous grid patterns having mixing fins are already known. French Specification No. 2,198,221, for example, discloses a grid comprising fins which generate a vortex having a large diameter with respect to the spaces between fuel elements. French Specification No. 2,093,981 describes a grid whose fins are placed so as to induce a flow in the form of concentric currents, in alternating direction in the assembly. German Published Application No. 1,564,697 comprises half-fins having a distribution which induces a specific flow pattern but requires locating two half-fins at certain crossings while other crossings between two plates are completely devoid of fins.
To facilitate description of the invention and avoid any ambiguity, the following terminology will be adopted hereafter.
The term "constriction" will designate the position where two adjacent fuel elements are separated by the shortest distance. "Hydraulic cell" will designate the space between four constrictions defined by four adjacent fuel elements, whose axes occupy the nodal points of a square network. "Radial hydraulic balance" will designate the arithmetical sum of the flow rates which enter the hydraulic cell and leave it in the radial (as opposed to longitudinal) direction. A desired balance corresponds to a zero sum of the flows entering and leaving the cell perpendicularly to the general flow direction along the assembly.
The grids described in French Specifications Nos. 2,198,221 and 2,093,981 are unsatisfactory in that they generate a flow exhibiting a non-zero hydraulic balance in each cell. In addition, the volume of material constituting the fins changes from one hydraulic cell to the other in the grid described in French Specification No. 2,198,221 and German Published Application No. 1,564,697.
French Patent No. 1,536,256 describes a grid comprising fins placed in the constrictions. This choice is hydraulically unfavorble, for it further reduces the cross-sectional flow area offered to the coolant at a place where that area is already at a minimum.
Finally, French Patent No. 1,536,258 describes a grid comprising fins placed at the intersections of the crossed strips forming the grid. But, due to the longitudinal bulk of the fins, it is necessary to place them partly on one side of the grid and partly on the other side. Furthermore, the flow induced in the constrictions is very complex. The flow occurs from one grid to the next in the longitudinal direction of the assembly, in a form which, as seen in a plane perpendicular to the general flow, is in the form of crenellations. Finally, the radial hydraulic balance is not zero in the cells.